FT232R datasheet(24/43 Pages) FTDI

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Document No.: FT_000053

FT232R USB UART IC Datasheet Version 2.07

Clearance No.: FTDI# 38

6.2 Self Powered Configuration

Figure 6.2 Self Powered Configuration

Figure 6.2 illustrates the FT232R in a typical USB self powered configuration. A USB self powered device

gets its power from its own power supply, VCC, and does not draw current from the USB bus. The basic

rules for USB self powered devices are as follows –

A self powered device should not force current down the USB bus when the USB host or hub

controller is powered down.

ii)

A self powered device can use as much current as it needs during normal operation and USB

suspend as it has its own power supply.

iii)

A self powered device can be used with any USB host, a bus powered USB hub or a self

powered USB hub.

The power descriptor in the internal EEPROM of the FT232R should be programmed to a value of zero

(self powered).

In order to comply with the first requirement above, the USB bus power (pin 1) is used to control the

RESET# pin of the FT232R device. When the USB host or hub is powered up an internal 1.5kΩ resistor on

USBDP is pulled up to +3.3V (generated using the 4K7 and 10k resistor network), thus identifying the

device as a full speed device to the USB host or hub. When the USB host or hub is powered off, RESET#

will be low and the FT232R is held in reset. Since RESET# is low, the internal 1.5kΩ resistor is not pulled

up to any power supply (hub or host is powered down), so no current flows down USBDP via the 1.5kΩ

pull-up resistor. Failure to do this may cause some USB host or hub controllers to power up erratically.

Figure 6.2 illustrates a self powered design which has a +4V to +5.25V supply.

Note:

1. When the FT232R is in reset, the UART interface I/O pins are tri-stated. Input pins have internal

200kΩ pull-up resistors to VCCIO, so they will gently pull high unless driven by some external

logic.

2. When using internal FT232R oscillator the VCC supply voltage range must be +4.0V to 5.25V.

3. When using external oscillator the VCC supply voltage range must be +3.3V to 5.25V

Any design which interfaces to +3.3 V or +1.8V would be having a +3.3V or +1.8V supply to

VCCIO.

FT232R Datasheet(PDF) 24 Page - Future Technology Devices International Ltd.